As a thermal transfer sheet used for formation of images using thermal transfer, a sublimation thermal transfer sheet in which a dye layer consisting of a thermal diffusion dye (sublimation dye) is provided on a substrate sheet of a plastic film or the like, and a thermofusible thermal transfer sheet in which an ink layer consisting of a pigment and a wax is provided in place of the dye layer are known. These thermal transfer sheets form images by being heated from the backside by a thermal head and transferring a dye in the dye layer or a pigment in the ink layer to a material to which the dye or the pigment is transferred.
In these thermal transfer sheets, when a substrate film is a thermoplastic film, if the formation of images is performed at the thermal head, the surface intrinsic resistivity of a contact surface of the film with the thermal head becomes high, and therefore there was a problem that static electricity is generated and the sheet is apt to be charged. Particularly in the case of the thermofusible thermal transfer sheet, since most of the ink layer is predominantly composed of a wax, this sheet tends to be charged. When the thermal transfer sheet is charged, dust settles on the surface of the thermal transfer sheet or the thermal head and problems that the resolution of images to be formed is deteriorated and that feeding property of the material to which the dye or the pigment is transferred is deteriorated due to charging of the material such as paper.
In order not to cause these problems, hitherto, means for imparting an antistatic property to the thermal transfer sheet has been investigated.
As the means to impart an antistatic property to the thermal transfer sheet, for example, it is known to provide a primer layer containing an antistatic agent (conductive material or the like).
As a thermal transfer sheet provided with the primer layer containing an antistatic agent, a thermal transfer sheet, in which a primer layer containing sulfonated polyaniline as an antistatic agent and a hardening resin such as a polyester resin is provided on the face opposite to a face of the substrate sheet on which a thermally-transferable color material layer is provided with the primer layer interposed between a heat resistant slipping layer and the substrate sheet, is proposed in Patent Document 1.
As a thermal transfer sheet using the same antistatic agent in the same layer constitution as in the sheet described in Patent Document 1, a thermal transfer sheet, in which a binder resin composing the primer layer is limited to a resin (for example, a water-dispersible or water-soluble polyester resin) having specific viscoelasticity, is proposed in Patent Document 2.
As a thermal transfer sheet provided with the primer layer containing an antistatic agent, a heat-sensitive transfer material, which is formed by providing a primer layer of low resistance formed by dispersing a powder of metal oxide such as indium or tin as a conductive agent in a binder resin between the substrate and a thermofusible ink layer, is described in Patent Document 3.
However, in the above technologies of forming a conductive primer layer by use of the conductive agent (antistatic agent) and the binder resin, if the heat resistance of the binder resin is inadequate, there are problems that the primer layer is rubbed off or print wrinkles are produced due to the heat of a thermal head during printing images, and furthermore there is a problem that compatibility between the conductive agent and the binder resin have to be considered and a combination of materials is restricted.
As a thermal transfer sheet prepared by forming a conductive layer using specific metal oxides, a heat-sensitive transfer recording medium, in which a transparent ceramic vapor deposition layer formed by depositing the specific metal oxides by vacuum deposition is provided on the face opposite to a face of a polyester film substrate on which an ink layer is provided as a heat resistant treatment layer, is proposed in Patent Document 4. In Patent Document 4, only TiO2 is used in Examples, but Al2O3 is exemplified as the specific metal oxide.
Although this recording medium provided with the ceramic vapor deposition layer has heat resistance, a special apparatus is necessary for vapor deposition and there is a problem that a production cost becomes high.
Patent Document 1: Japanese Kokai Publication 2000-272254
Patent Document 2: Japanese Kokai Publication 2001-1653
Patent Document 3: Japanese Kokai Publication Hei-2-20390
Patent Document 4: Japanese Kokai Publication Hei-8-267942